<!DOCTYPE html>
<html>
<body>
<ul id="tree" role="tree" tabindex="0" aria-activedescendant="before">
  <li id="before" role="treeitem" tabindex="0" aria-expanded="false">
    Major
  </li>
  <li role="treeitem" tabindex="0" aria-expanded="true">
    Minor
  </li>
  <ul role="group">
    <li role="treeitem" tabindex="0">
      Minor Seventh
    </li>
    <li id="after" role="treeitem" tabindex="0">
      Minor Ninth
    </li>
  </ul>
</ul>
<script>
  var tree = document.getElementById('tree');
  tree.focus();
  function go() {
    tree.setAttribute('aria-activedescendant', 'after');
  }
</script>
</body>
</html>
